1. Field of the Invention
The present invention relates to a leakage diagnosing device for an evaporated gas purge system for diagnosing a leakage (pressure leakage) in an evaporated gas purge system which purges (discharges) evaporated gas, caused by a fuel evaporation in a fuel tank, to an intake pipe of an internal combustion engine.
2. Description of Related Art
According to a conventional evaporated gas purge system, in order to prevent evaporated gas generated from inside of a fuel tank from leaking out to the atmosphere, the evaporated gas is adsorbed in a canister via an evaporated gas passage at the inside of the fuel tank. A purge control valve is installed at a midway of a purge passage for purging the evaporated gas, adsorbed in the canister, to an intake pipe of an internal combustion engine. An opening and closing of the purge control valve is controlled in accordance with an operating state of the internal combustion engine to control a purge flow rate of the evaporated gas purged from the canister to the intake pipe. In order to prevent extensive leakage of the evaporated gas from the evaporated gas purge system to the atmosphere, the leakage of the evaporated gas needs to be detected at an early stage.
According to a conventional diagnosing method disclosed in, for example, JP-A-5-125997, an existence or an absence of leakage is diagnosed based on a pressure change amount in a gas purge passage after introducing the negative pressure of an intake pipe into the gas purge passage and sealing the gas purge passage including a fuel tank and a canister hermetically. The pressure change amount in the gas purge passage is detected by a pressure sensor, and is compared to a predetermined value.
However, the pressure change amount after introducing the negative pressure fluctuates when the introduced negative pressure changes, even if the gas purge passage is normal (no leakage). In other words, the pressure change amount after the negative pressure introduction may increase as the introduced negative pressure becomes lower. Therefore, a leakage diagnosis error may be caused by such fluctuation of the pressure change amount when only the pressure change amount is used as a diagnosis parameter for the leakage diagnosis.
The leakage diagnosis error may also be caused by, not only such fluctuation of the pressure change amount, but also diagnosis timing, a fluctuation of a fuel condition parameter such as a remaining amount of fuel or fuel temperature, a generated amount of the evaporated gas, and the like.